1. Field of the Invention
This invention relates to a method for electrostatic assistance in printing processes, and printing machines having electrostatic substrate contact pressure.
2. Description of the Prior Art
It has been known for a long time that an electrostatically charged foil being of material which is not electrically conductive is drawn onto a surface having an oppositely polarized charge until the charge has been equalized. The present invention makes use of this physical effect in order to press a printing substrate--eg. a sheet or strip of paper, plastic or textile material--firmly and bubble-free onto the printing cylinder while said substrate passes through the printing gap between the printing cylinder and the contact pressure roller. The GB-PS No. 11 59 923 describes a corresponding printing machine in which a corona wire lying opposite the contact pressure roller is used as electrode arrangement.
The transfer of ink drops from the printing cylinder to the paper is also improved by the electrostatic field applied, since the ink droplets are set in lively motion, fall onto the paper and penetrate into it more quickly than is the case with simple surface contact. Due to the electrostatic contact pressure of the substrate, which should not exhibit too high a dielectric constant, the quality of the printing process can be improved and also a speeding up of the same can be achieved. It is however almost impossible to put these advantages into effect, since in cases of unavoidable tearing of the substrate during operation the high corona charge necessary for efficient working leads to the formation of spark discharges, which ignite the explosive mixture of air and printing ink solvents surrounding the printing machine and thus trigger off an explosion. Despite many attempts this problem has not yet been solved satisfactorily enough to meet the requirements set by the corresponding supervisory authorities for the industrial use of such printing machines being provided with electrostatic substrate pressure devices.